This invention relates generally to power converters and more particularly, to power converters that enable variable speed operation of electrical machines at very high power and relatively high frequency.
Semiconductor switches capable of handling high currents and voltages (high currents and voltages typically are in excess of 200 amps and 500 volts) typically cannot be switched at high frequency (high frequencies typically are in excess of 5000 Hertz due to thermal limitations. As a result, harmonic content of power processed by high power converters is high and significant filtering generally must be performed before the power signal is applied to an electrical machine or to a power grid. Such filtering adds to the costs and adversely impacts the efficiently of such converters.
As one example application, high power typically is required by motor drives. With motor drives, however, torque ripple may be generated by the drive, which can result in harmonic content in the power applied to the motor. In addition, noise injected into the power grid may result in further degradation in the power applied to the motor. Such harmonic content and noise can lead to degradation of various components sensitive to low cycle fatigue.
One known converter configuration includes multiple single-phase converter modules connected in series to an attempt to increase the voltage level and improve power quality by adding more levels in the voltage waveform. This type of configuration is commonly referred to as a series stacked converter. With such a series stacked converter, however, harmonic content also is passed in series from module to module, and significant filtering typically is performed before the resulting power signal is applied to an electrical machine or to a power grid.